The purpose of this project is to operate, maintain and provide pedigreed, defined microflora associated rodent foundation colonies for supplying breeders to the expansion colonies of the rodent production centers. Offspring from genetically defined inbred pedigreed stock obtained from NIH Repository or other sources will be Cesarian derived, maintained in isolators and associated with defined microflora. Offspring from these foundation stock will be used to supply breeders to the expansion colonies of new rodent production colonies at a rate of 30 breeding pairs of Fischer 344/N rats and 45 breeding pairs of C57BL/6N and C3H/HeN MTV- mice and appropriate number of breeding pairs of other strains per month. Fishcer 344/N rats and B6C3F1 hybrid mice are the selected experimental animal species for evaluation of chemicals for toxicologic and carcinogenic properties under DTRT/NTP. The genetic integrity of the apparent generations of Fischer 344/N rats and the C57BL/6N and C3H/HeN MTV- mice to produce the B6C3F1 hybrid is essential for developing reliable and accurate research data in animal experiments. Centralized foundation colonies will provide better control on genetic integrity of all the breeders in the expansion and production colonies of rodent production centers.